Reflow soldering - Part (4) First soldering test

Plot of actual temperature change

The board I used for the test, SMD 1206 resistor centre

I have been experimenting with the oven under the control of my programmable temperature controller, ( see 12 December ), and making minor changes to the software until I obtained the heating and cooling characteristic above. It closely resembles the ideal profile, ( see 8 October ).
Eager to try some soldering, I put several blobs of solder-paste and a surface-mount resistor on a scrap piece of copper clad circuit board material, placed it in the oven and started the process. After completion I inspected the board. The solder-paste had become metallic, the resistor was nicely soldered and the copper hadn't 'lifted'.
Without any stencil, I had applied far too much solder-paste.

Reflow soldering - Part (3) Temperature controller

Controller unit with SSR and thermocouple wire connected

Internal view - MAX6675 on mini board alongside PIC

I have made another oven temperature controller in a convenient to use enclosure, which has a sloping surface for the back-lit liquid crystal display module. It looks quite professional. The circuit is the same as the earlier experimental version, ( posted on 11 November ), using a PIC microcontroller and MAX6675 temperature sensor chip. It is ready for integrating with the oven. The output of the 15 amp solid state relay ( SSR ) has to be connected in series with the mains lead to control the power. The thermocouple will be situated inside the oven eventually very close to, or actually touching, the circuit board which is being soldered.
I am thinking that it would be beneficial to add a programming interface to the controller so that I can re-program the embedded PIC in-situ.

Digital thermometer

Arduino 'Uno' board and temperature sensor board

L: MCP2210 USB to SPI; upper R: LM35D; lower R: MAX6675

My fun with measuring temperature continues while waiting for the solder-mask stencils for reflow-soldering to be prepared. I have been using the LM35D temperature sensor chip. It doesn't require a thermocouple as the chip itself produces a voltage related to its own temperature, ( 10mV per degC ), and because the chip generates almost no heat, its temperature is nearly identical to that of its immediate surroundings. It's accurate to within about 0.5C. This time, because my DIY development board, ( MYDEV2 ), is otherwise engaged, I have been using my Arduino 'Uno' board with Atmel Atmega328P-PU microcontroller. I use the Arduino's analogue to digital converter to sample the voltage produced by the LM35D and then I process the 10 bit digital value to get the temperature for displaying. For no reason other than to use the second row on the display, I've also added a timer to count the seconds which have elapsed since the last program upload or reset.

I might make another for use as the workshop's digital thermometer.

Reflow soldering - Part (2) Measuring temperature

Thermocouple and serial data connections
MAX6675 on underside of top board

Temperature in my electronics lab

Since posting Part (1) on 8 October I can now measure temperature with 0.25C resolution using the MAX6675 temperature sensor chip and a thermocouple. I wrote some code for a PIC microcontroller's MSSPI to receive the 16 bits of serial temperature data from the MAX6675, display a value and update it every second. The display also shows the phase in progress during the reflow-soldering process, e.g., ramp to soak.

The next step of this project is to use the temperature measurement as the basis for controlling the rate of temperature change inside the oven. I have the plots made earlier of the temperature change at various power duty-cycles to the elements and I shall be using these at the starting point for each phase of the process

Follow up to the USB - TTL UART

Docklight running

In the post dated 31 July 2013 about the USB-TTL UART interface, I described how I tested the interface using my PIC development board. A much simpler method requiring only the circuitry with the FT232RL chip is a loopback test arranged by connecting together the TTL TX and RX lines. Using the serial comms analysis software called "Docklight" a complete sequence can be set up before sending. I also configured Docklight to repeat the transmit sequence every 5 seconds; in image above TX data red, RX data green, timestamp and comments yellow.

Reflow soldering - Part (1) Oven trials

Reflow soldering is a way to solder all surface mount components onto a circuit board simultaneously. I would find this very useful, and as I don't want to buy an industrial reflow soldering system, I considered a DIY approach.
The first step was to choose a mini electric convection oven to provide the necessary heat. A new Adler model AD6003, 1000 watt, 9 litre oven was purchased for only $35.

Adler AD6003 "toaster-oven", cover removed

The guarantee was immediately invalidated by removing its cover to expose the thermostat and mechanical 60min timer/bell, and then disabling both of them. The next task was to check that the temperature would at least reach the solder reflow point of about 217C for Pb/Sn solder. With the oven switched to maximum power, ( upper and lower heating elements ), this temperature could be exceeded by a sufficient margin. To obtain accurate measurements a thermocouple, temperature data logger and a laptop running a control program written in Visual Basic were used.
After adding some electronics to operate a solid state relay connected to the heating elements, several plots of temperature versus time during heating up at different power duty cycles were obtained. Ultimately, the aim is to achieve, to a close approximation, the idealised heating and cooling profile shown below.

Idealised thermal profile

This project is still a 'work-in-progress'. A Part (2) will be posted later.

Analog Devices AD9850 frequency synthesiser

AD9850 evaluation board, 45mm x 26mm

Connected to MYDEV2 PIC MSSP module for programming

SINA and QP outputs

Another visit to an online auction site and another electronics purchase. This time I spent $9 on an evaluation board for the AD9850 frequency synthesiser chip. Surely the 125MHz 'can' oscillator and the chip itself are individually worth more than that. However it was made in China.
I mounted it on a larger piece of experimenter board and connected its programming inputs to a microcontroller PIC18F4550 on my MYDEV2 PIC development board. Before the AD9850 will produce an output signal it has to be programmed.
So I wrote a few lines of code to use the PIC's Master Synchronous Serial Peripheral ,( MSSP ), interface module to send the 40 bits of frequency, phase and control data to the AD9850.
The resultant output signals are a sine wave ( CH1 yellow trace ) of 1.04V peak-peak directly from the chip's digital-analogue convertor, ( DAC ), and a variable pulse-width square wave ( CH2 blue trace ) of 5V peak-peak via the chip's comparator for use as an external clock.
I have intentionally allowed plenty of space on the experimenter board to fit a dedicated PIC later; probably the PIC18F14K22 as I already have one.
The AD9850 will be a useful signal source and clock generator upto about 40MHz.

USB - TTL UART

FT232RL board, USB left, TTL right

My descriptor embedded in the FT232RL

Serial comms setup

I had been looking for a usb interface solution for a piece of home-made gear when an old friend, who is a professional electronics engineer, reminded me of FTDI's range of interface chips. He even gave me one to play with; the 28 pin FT232RL. It's a USB-TTL UART device. So I did some experiments to practise using it.
First I made a circuit to use it as a usb powered device. After installing the drivers on a pc, I could see another usb(com) port had been detected. Then with a few more components added to the circuit I used it in my intended application as a self-powered usb device.
I programmed the user area of the chip's eeprom with my application data and some configuration options, e.g., allocating a couple of pins as outputs for LEDs. I was pleased to see that the same data I had just programmed in now appeared in the usb connection properties window.
Confirmation that the circuit was functioning correctly came when the red and green LEDs flashed in response to sent and received data between the PIC EUSART on MYDEV2 development board and the host pc running serial communications software.

More details of my low power transmitting setup for longwave

Since the posts on 22nd February and 8th May, I have received requests to post more information on the setup I used for my low power test transmissions on the longwave 2190m band.

The circuit schematic and pcb artwork for the AF amplifier are shown above; click on the images to expand them. The original size of the artwork is 70 x 100mm. The pcb is single-sided; top component layer, bottom copper layer. Anyone wishing to copy my pcb design might need to modify the tracks connecting T1, depending on the actual transformer which is available and the windings used.

A +18V dc power supply can be used for greater output power. I didn't try this only because I don't have a convenient way of providing that voltage, and also the fan is a 12V unit.

My very low power transmissions on longwave

Last night I made successful radio test transmissions on 137.7KHz, 2190m band, using only 3.5W transmitter power. My signal was received, ( screen capture below ), at a distance of 17Km. The signal strength suggests that 2-way communication at this power level would be possible over a much longer distance. The vertical streaks are probably static crashes as a thunder storm was active in the vicinity.

My setup was my own-design PIC controlled DDS and the TDA2030 AF amplifier featured on 22 February. 

It is unfortunate that amateur radio activity on the 2190m band is so low, as it is possible to enjoy communicating on this band with a minimal setup, as I have just shown.  

Cheap watts on 70MHz

A visit to an on-line auction site on 22nd October 2012 resulted in my purchasing, ( for about $18 ), an ex-commercial equipment radio frequency linear amplifier covering the band 60MHz - 80MHz, and probably capable of producing an output power of at least 100 watts. Very useful, I thought, for the amateur 70MHz VHF band.
I have no clue about the manufacturer. With the amplifier, however, came a much simplified hand-drawn circuit schematic with some notes in Russian.

To make it operational I mounted it inside a box, provided 24V, 12V and nominal 6V supplies, status LEDs, antenna changeover switching and a 7-pole Chebychev low-pass filter on the output.

 How smart it looks and it works too. What a bargain !

Experimental low power amplifier for 2190m longwave

I salvaged some potentially useful parts from a faulty pc power supply, e.g. bridge rectifier, schottky diodes, heatsink, fan, chokes, transformers. The 12V-0-12V, 5V-0-5V output transformer typically operates near 40KHz. I thought of using it for the output matching transformer in a low power transmit amplifier for the 136KHz, 2190m longwave band.
My design is based on the very cheap, ( half a $ ), TDA2030 class AB audio amplifier ic, which has a bandwidth of 140KHz.
The circuit is experimental. I was curious to find out if such an amplifier would be useful for 136KHz, despite using some untypical, possibly 'unsuitable', components.

I built the amplifier on a home-made printed circuit board, 70 x 100mm. The ex-pc transformer, ( yellow & black ), is on the left. The TDA2030 is mounted on the ex-pc heatsink. ( Pcb artwork and the circuit schematic are available from me on request ).

Fitting the circuit board inside the old pc power supply box, ( cover not shown ), with its original 12V fan, and adding a LED, rf and dc connectors, completed the construction.

For testing, I powered the amplifier from a +13.6Vdc power supply and connected the input to my frequency synthesiser tuned to 137.8KHz. With the input attenuation set to minimum, and the output terminated in a 50 Ohm load, the measured voltage gain was 41.75dB. Output power was 3.5W.
I could now either connect the amplifier directly to my longwave antenna and make some very low power test transmissions, or use it as an intermediate amplifier stage in a much more powerful transmitter, yet to be built.

Chirp-Hell on longwave

On 14th November 2012 I reported success with the tests on the work-bench of the improved ssb phasing exciter for my 2190m longwave transmitter. Soon after that I installed it inside the transmitter enclosure. Since then I had been waiting for an opportunity to test it using full transmitter power into my antenna in a real 'on-air' situation, with a more distant receiver. 

So early this morning at about 1.00am I carried out transmission tests with Jacek, SQ5BPF, in Warsaw. My signal was quite readable on his grabber; screen captures above. The noisy band conditions at his location are also very evident.

I was transmitting on 136.9KHz upper sideband, modulating with 800-810Hz chirp-hellschreiber audio tones. Transmission speed was either 5 or 10 secs/character; the vertical markers are 1min apart. We then completed a chirp-hell to qrss1 cross-mode contact; quite obscure, so it's probably the first time ever it has been done !